1. Field of the Invention
This is a continuation-in-part of application Ser. No. 875,443, filed June 17, 1986, now abandoned.
The present invention relates to co-vulcanizable compositions of fluoroelastomers.
More particularly, the present invention relates to co-vulcanizable compositions of fluoroelastomers having improved chemical stability.
2. Description of the Prior Art
Fluoroelastomers on the basis of vinylidene fluoride, vulcanized according to the bisphenolic or dihydroxy systems (Kirk-Othmer, Encyclopaedia of Chemical Technology, Vol. 8, page 506, 1979), are very valuable products which are used in the industrial world, in particular in the fiels of the automotive and oil industries, and in general in the industrial energy field.
In the automobile field, fluoroelastomers are used mainly in the fuel circulation system or as components of engine parts and of parts of gearbox-differential-transmission units. In the first case, as application examples, membranes for fuel pumps, valve seats, non-return valves flexzible hoses and, in general, sealing gaskets for fuel can be mentioned. In the second case, analogous examples are gaskets for gear wheels, sealing liners for cylinders in diesel engines, gaskets for the crankshaft, non-return valves for the exhaust gases, gaskets of the type of shaft seals, and still others.
In the oil and industrial energy field, fluoroelastomers are used for applications wherein they come into direct contact with crude oil; and above all at high temperatures and pressures, such as, for example, for gaskets, valves and pumps, or parts thereof. In this field, an important application for fluoroelastomers is represented by the expansion joints used in the exhaust ducts for flue gases in thermoelectric power plants, wherein resistance to high temperatures combined with resistance to the corrosive action of acidic flue gases is required.
Fluoroelastomers are used in these, and in other fields, such as in the electric and electronic fields, thanks to their properties of high thermal stability and chemical resistance, even at high temperatures.
But during recent years, and in some application types, precise technological requirements have arisen which the manufactures of the prior art, vulcanized by the bisphenolic or dihydroxy systems, are not able to fullfil.
The problem involves in particular the chemical stability under severe operating conditions characterized by high temperatures in the presence of steam, of mineral acids, of highly aggressive lubricating fluids, such as, e.g., the oils of the SF series, containing as additives compounds having a high swelling capacity for fluoroelastomers based on vinylidene fluoride.
Furthermore, a general improvement is required as regards the processability characteristics of the mixes, for the molding of such articles as shaft seals, as well as the possibility to easily extrude the mixes to produce suitable semifinished articles which may even have complex profiles.
The problem of the chemical stability has been solved thanks to the use of the peroxidic vulcanization process.
However, this type of vulcanization does not fulfill the above-mentioned requirements as regards processability, in particular by molding and extrusion; furthermore, the same processability turns out to be less reliable (Rubber Chemistry and Techology, Vol. 55, page 906, 1982), and on the compression set test much poorer results are obtained (Kirk-Othmer, ibid., page 510).
In European patent application No. 84.110707, covulcanized compositions have been prepared and disclosed which have allowed certain improvements in performance to be achieved, but which still show poor processing reliability, as well as a slight decrease in chemical-thermal properties.